Method and device for cryocondensation

ABSTRACT

A method and device for cooling and/or purifying a process gas, in which the process gas is precooled in a precooler by indirect heat exchange with a second coolant stream, and then cooled in a deep-freeze unit in indirect heat exchange with a first coolant stream. The first process stream is generated by vaporizing a condensed gas. At least one part of the first coolant stream leaving the deep-freeze unit is sent as at least part of the second coolant stream to the precooler, and at least another part of the first coolant stream leaving the deep-freeze unit is used with at least one part of the second coolant stream leaving the precooler to vaporize the condensed gas is vaporized and is vaporized. The part of the first coolant stream used to vaporize the condensed gas may be the part leaving the deep-freeze sent to the precooler.

This application claims the priority of German Patent Document No. 10 2005 033 252.8, filed Jul. 15, 2005, the disclosure of which is expressly incorporated by reference herein.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a method of cooling and/or purifying a process gas, whereby the process gas is precooled in a precooler in indirect heat exchange with a second coolant stream and then is refrigerated in a deep-freeze unit in indirect heat exchange with a first coolant stream, whereby at least one part of the first coolant stream leaving the deep-freeze unit is sent to the precooler as a second coolant stream and whereby the first coolant stream is obtained by vaporizing a condensed gas.

Furthermore, the invention relates to a device for cooling and/or purifying a process gas, having a precooler designed as an indirect heat exchanger with an inlet line for process gas and an outlet line for precooled process gas and with a coolant inlet line and a coolant exhaust, with a deep-freeze unit designed as an indirect heat exchanger having an inlet line for process gas and an outlet line for deep-cooled process gas and with a coolant inlet line and a coolant exhaust, with an evaporator designed as an indirect heat exchanger with a coolant inlet line and a coolant exhaust, whereby the outlet line of the precooler for precooled process gas is connected to the inlet line of a deep-freeze unit for process gas, whereby the coolant exhaust of the evaporator is connected to the coolant inlet line of the deep-freeze unit, and the coolant exhaust of the deep-freeze unit is connected to the coolant inlet line of the precooler.

In purification of a process gas by cryocondensation, the process gas to be purified is brought into indirect heat exchange with a deep-cooled coolant in a heat exchanger to freeze or condense impurities out of the process gas. In this process it is important for the heat transfer between the coolant and the process gas not to be too intense. Thus, it is often not desirable to use liquid nitrogen directly as a coolant.

However, liquid nitrogen is basically an advantageous refrigerant. Therefore, the processes used prevent the passages for liquid nitrogen from being directly adjacent to the passages for the process gas in the heat exchanger. For example, it is known from DE 40 01 330 A1 that liquid nitrogen may be vaporized in the central tube of a double tube, with the resulting gaseous nitrogen carried in the surrounding concentric annular gap. The process gas to be purified and/or cooled flows past the double tube on the outside and is cooled by the gaseous nitrogen in the process.

German Patent DE 102 23 845 C1 proposes the use of a refrigerant cycle which transfers cold from liquid nitrogen to process gas.

European Patent EP 0 749 549 B1 describes a method of cooling a process gas in which liquid nitrogen is first vaporized in a preevaporator and the resulting gaseous nitrogen is then used for cooling the process gas.

To better utilize the refrigeration content of the coolant, it is also known that the coolant leaving a first heat exchanger, a deep-freeze unit, may be sent to a second heat exchanger, which functions as a precooler, as a refrigerant medium. The process gas to be cooled is then carried in countercurrent first through the precooler and then through the deep-freeze unit.

In practice, there are often relatively great fluctuations in the process gas stream. This has marked effects on the heat transfer between refrigerant and process gas. In particular, when the process gas is to be purified first in a precooler and then a deep-freeze unit, this may result in the precooler not making any noteworthy contribution toward condensation, and the entire purification, i.e., condensation or freezing out, takes place in the deep-freeze unit. There is the risk here that impurities in the process gas may freeze out on the heat exchanger surfaces of the deep-freeze unit and thereby block these surfaces.

The object of the present invention is therefore to provide a method and a device of the above-defined type, such that a better cooling power and purification power are achieved even with a variable volume flow of process gas to be cooled or purified.

This object is achieved by a method of cooling and/or purifying a process gas

-   -   whereby the process gas is precooled in indirect heat exchange         with a second coolant stream in a precooler,     -   and is cooled with a first coolant stream in indirect heat         exchange in a deep-freeze unit,     -   whereby at least one part of the first coolant stream leaving         the the deep-freeze unit is sent to the precooler as the second         coolant stream,     -   whereby the first coolant stream is obtained by vaporizing a         condensed gas, and

whereby the condensed gas is vaporized in indirect heat exchange with at least one part of the first coolant stream leaving the deep-freeze unit and with at least one part of the second coolant stream leaving the precooler.

The inventive device for cooling and/or purifying a process gas comprises

-   -   a precooler designed as an indirect heat exchanger having a         process gas inlet line and a process gas outlet line and having         a coolant inlet line and a coolant exhaust,     -   a deep-freeze unit designed as an indirect heat exchanger having         a process gas inlet line and a process gas outlet line and         having a coolant inlet line and a coolant exhaust,     -   an evaporator designed as an indirect heat exchanger having a         coolant inlet line and a coolant exhaust and having a heating         medium inlet line and a heating medium exhaust,     -   whereby the process gas outlet line of the precooler is         connected to the process gas inlet line of the deep-freeze unit,     -   whereby the coolant exhaust of the evaporator is connected to         the coolant inlet line of the deep-freeze unit,     -   whereby the coolant exhaust of the deep-freeze unit is connected         to the coolant inlet line of the precooler, and     -   whereby the coolant exhaust of the deep-freeze unit and the         coolant exhaust of the precooler are connected to one of the         heating medium inlet lines of the evaporator.

According to this invention, a condensed gas is vaporized in an evaporator. The resulting cold gas is then used as the first coolant stream in the deep-freeze unit. The first coolant stream removed from the deep-freeze unit is sent at least partially as a second coolant stream to a precooler.

The process gas to be cooled is first passed through the precooler and thereby precooled. A portion of the impurities present in the process gas is already condensed or frozen out in the precooler. Downstream of the precooler, the precooled process gas is directed into the deep-freeze unit and cooled to the desired final temperature and/or the remaining impurities are condensed out or frozen out.

According to this invention, the process gas is subject to a stepwise cooling, namely first in the precooler and then in the deep-freeze unit, using vaporized gas as the coolant to prevent the problems mentioned in the introduction, which may occur when using deep-cold liquid coolants.

The enthalpy of vaporization for vaporizing the condensed gas is supplied by two streams. Firstly, at least one part of the first coolant stream taken from the deep-freeze unit and already partially heated is sent to the evaporator as a heating medium; secondly, the second coolant stream coming from the precooler is partially used as a heating medium.

According to this invention, the enthalpy of vaporization required for vaporizing the condensed gas is obtained not from one stream alone but from two streams. The two streams may be combined upstream of the evaporator and sent as a combined stream to the evaporator or, preferably, as separate streams into and through the evaporator.

Preferably neither the complete first coolant stream from the deep-freeze unit nor the complete second coolant stream from the precooler is sent to the evaporator, but instead only a portion of the each of these two coolant streams is sent. However, it is also possible to send the complete first coolant stream leaving the deep-freeze unit to the evaporator and, in addition, to also use the second coolant stream for vaporizing the condensed gas. Conversely, the complete second coolant stream and only a portion of the first coolant stream may of course also be used.

An essential aspect of the inventive method is that two different streams are used as the heating medium for vaporizing the condensed gas. In this way, the heat transfer conditions in the evaporator, precooler and deep-freeze unit can be adapted to fluctuations in the process gas stream in a targeted manner. In particular, exactly the amount of heat required to vaporize the condensed gas sent into the evaporator may be sent to the evaporator.

Furthermore, the temperature conditions in the precooler may be adjusted, i.e., regulated. The part of the first coolant stream leaving the deep-freeze unit and being sent to the evaporator as a heating medium is subjected to intermediate cooling by indirect heat exchange with the condensed gas in the evaporator. The temperature in the precooler is established according to which volume stream of the first coolant stream has been through intermediate cooling and then sent to the precooler.

The quantities of the first and/or the second coolant stream used for vaporizing the condensed gas are preferably determined as a function of the temperature of the vaporized condensed gas. To do so, the temperature is preferably measured on the coolant exhaust of the evaporator, where the vaporized gas escapes, and the quantities of the two coolant streams supplied to the evaporator as a heating medium are regulated as a function of this temperature.

Furthermore, a setpoint value for the inlet temperature of the second coolant stream into the precooler is especially preferably preselected. Then the first and second coolant streams, in particular the parts of these coolant streams supplied to the evaporator, are regulated as a function of this setpoint value and as a function of the outlet temperature at the coolant exhaust of the evaporator.

Fluctuations in the process gas stream have direct effects on the heat transfer conditions in the deep-freeze unit and in the precooler. The temperature of the first coolant stream leaving the deep-freeze unit and the temperature of the second coolant stream leaving the precooler depend on the quantity and composition of the process gas and the operating conditions. The vaporization in the evaporator is adapted to fluctuations in the process gas stream through the inventive method of regulating the coolant streams and optimum heat transfer conditions are ensured at all times in the precooler and the deep-freeze unit.

Nitrogen has proven to be the preferred coolant. Liquid nitrogen is supplied to the evaporator and vaporized. The resulting gaseous nitrogen is sent first to the deep-freeze unit and then to the precooler. A portion of the nitrogen gas leaving the deep-freeze unit is used in the evaporator for vaporizing the liquid nitrogen and then sent to the precooler. The enthalpy of vaporization for vaporizing the liquid nitrogen is also applied by a substream of gaseous nitrogen diverted at the outlet of the precooler.

The inventive method is suitable for cooling a process gas stream and in particular for purifying a process gas stream. For purifying the process gas stream, the heat transfer conditions and temperature conditions prevailing in the precooler and/or in the deep-freeze unit are selected so that vaporized substances and impurities present in the process gas stream are condensed or frozen out. This procedure is advantageous when the goal is to obtain a purified process gas stream as well as when substances present in the process gas stream are to be recovered.

Condensed impurities or substances are preferably removed from the precooler and/or deep-freeze unit. The passages of the precooler and/or deep-freeze unit through which the process gas that is to be purified and cooled flows are advantageously to be provided with a corresponding condensate outlet for this purpose. A collecting tank is preferably also provided, with the condensate drains opening into it for collection, recovery and optional recycling of the condensate.

At least one part of the first coolant stream removed from the deep-freeze unit is sent as a heating medium to the evaporator, according to this invention. The stream or at least one part of this stream is preferably sent as the second coolant stream to the precooler after leaving the evaporator. In the evaporator, intermediate cooling of the first coolant stream is performed by indirect heat exchange with the condensed gas before entering the precooler as the second coolant stream. Due to the intermediate cooling, the temperature of the second coolant stream may be regulated so that a certain portion of the impurities in the process gas stream is already condensed or frozen out in the precooler.

Accordingly, the inventive device is advantageously designed such that the heating medium exhaust, which is connected by the heating medium inlet line to the coolant exhaust of the deep-freeze unit, is connected to the coolant inlet line of the precooler.

For energy-related reasons, it is especially expedient to use the purified cold process gas stream leaving the deep-freeze unit for cooling the unpurified process gas stream supplied to the precooler. To this end, another cooler is connected upstream of the precooler, so the unpurified process gas stream is precooled in indirect heat exchange with the cold purified process gas stream.

This invention offers definite advantages in comparison with known methods. Due to the intermediate cooling of the first coolant stream leaving the deep-freeze unit, e.g., the gaseous nitrogen, a contribution to condensation is always made in the precooler even with variable process gas streams. The precooler is already used for exhaust gas purification for all throughput quantities of process gas. Therefore, this results in a more uniform load on the heat exchanger surfaces in the precooler and the deep-freeze unit. With initial freezing of the impurities in the process gas stream, the quantities of ice are better distributed over the heat exchanger surfaces of the precooler and deep-freeze unit. The cooling of the process gas stream takes place more slowly and more uniformly, so this reduces the risk of fog formation, for example. This method is flexibly adaptable to different process gas volume and process gas compositions, so that the purification performance of the equipment can be optimized. This invention has proven successful in particular for purification of process gases loaded with low-boiling vaporous constituents such as solvents. The vaporous constituents can be recovered with the inventive method, yielding a pure process gas stream.

Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing for example.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE shows a schematic diagram of an embodiment of the inventive process gas purification.

DETAILED DESCRIPTION

The FIGURE shows schematically a system for cryocondensation of volatile substances in a process gas stream 1. The process gas stream 1 contains vaporous impurities such as readily volatile solvents, which are to be removed from the process gas stream 1 and recovered.

The process gas stream 1 loaded with impurities is sent to a precooler 2, where the process gas stream 1 is cooled in indirect heat exchange with gaseous nitrogen and a portion of the impurities condenses out or is frozen out. The partially purified process gas stream 3 is then also cooled in indirect heat exchange with gaseous nitrogen in a deep-freeze unit 4. The purified process gas stream 5 is removed from the deep-freeze unit 4.

The condensate obtained in cryocondensation in the precooler 2 and in the deep-freeze unit 4 is removed from the precooler 2 and/or from the deep-freeze unit 4 through lines 6, 7 and sent to a storage container 8.

Liquid nitrogen 9 is taken from a tank (not shown in the FIGURE) and sent to an evaporator 10. The liquid nitrogen is vaporized in the evaporator 10 and the resulting gaseous nitrogen 11 is sent as coolant to the deep-freeze unit 4. The gaseous nitrogen 12 leaving the deep-freeze unit 4 is partially conveyed further to the precooler 2, where it also serves as a coolant for condensation of impurities contained in the process gas stream 1.

The remaining part 13 of the gaseous nitrogen 12 leaving the deep-freeze unit 4 is sent to the evaporator 10 and serves as a heating medium for using a portion of the heat of vaporization to vaporize the liquid nitrogen 9. In so doing, gaseous nitrogen 13 is cooled. After intermediate cooling, the nitrogen 14 is then combined with the nitrogen stream removed from the deep-freeze unit 4 and sent directly to the precooler 2, and the combined stream is sent to the precooler 2.

The quantity ratio between the nitrogen stream 13 sent to the evaporator 10 and the nitrogen stream sent directly to the precooler 2 can be adjusted via a valve 15 in the line connecting the deep-freeze unit 4 and the precooler 2.

Some of the gaseous nitrogen 16 partially warmed in the precooler 2 is removed from the process through line 17 and sent for use otherwise, e.g., as an inert gas. However, a portion 18 of the nitrogen stream 16 is also sent as a heating medium to the evaporator 10 for vaporizing the liquid nitrogen 9. The nitrogen 19 cooled in the process is then combined with the nitrogen stream 17 and removed.

The quantity 18 of the nitrogen stream 16 sent to the evaporator 10 can be adjusted and regulated via a three-way valve 20.

The streams 13 and 18 supplied to the evaporator 10 as a heating medium can be regulated as a function of the temperature of the vaporized nitrogen 11 leaving the evaporator 10. To this end, a temperature sensor 21 is provided and connected to the valve 15 by a signal line 22 and to the three-way valve 20 by a signal line 23.

The foregoing disclosures has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof. 

1. A method of cooling and/or purifying a process gas, comprising the steps of: generating a first coolant stream by vaporizing a condensed gas; precooling the process gas in a precooler by indirect heat exchange with a second coolant stream; and cooling the process gas in a deep-freeze unit by indirect heat exchange with the first coolant stream; wherein at least a part of the first coolant stream leaving the deep-freeze unit is sent to the precooler as at least a part of the second coolant stream entering the precooler, and the condensed gas is vaporized by indirect heat exchange with at least a part of the first coolant stream leaving the deep-freeze unit and at least one part of the second coolant stream coming from the precooler.
 2. The method according to claim 1, wherein quantities of the first and/or second coolant stream used for vaporization of the condensed gas are determined as a function of the temperature of the first process stream after indirect heat exchange to vaporize the condensed gas.
 3. The method according to claim 1, wherein the first coolant stream is obtained by vaporizing liquid nitrogen.
 4. The method according to claim 1, wherein impurities are condensed or frozen out of the process gas in the precooler and/or in the deep-freeze unit.
 5. The method according to claim 4, wherein impurities condensed out in the precooler and/or in the deep-freeze unit are removed from the precooler and/or from the deep-freeze unit.
 6. The method according to claim 1, wherein the part of the first coolant stream used for vaporizing the condensed gas is sent as the second coolant stream to the precooler.
 7. A device for cooling and/or purifying a process gas, comprising: a precooler in the form of an indirect heat exchanger for exchanging heat between the process gas a second coolant stream, the precooler having a process gas inlet line, a process gas outlet line, a coolant inlet line, and a coolant exhaust; a deep-freeze unit in the form of an indirect heat exchanger for exchanging heat between the process gas a first coolant stream, the deep-freeze unit having a process gas inlet line, a process gas outlet line, a coolant inlet line, and a coolant exhaust (12); and an evaporator in the form of an indirect heat exchanger for generating the first process stream by vaporization of a condensed gas, the evaporator having a coolant inlet line, a coolant exhaust, at least one heating medium inlet line, and a heating medium exhaust, wherein the precooler process gas outlet line is connected to the deep-freeze unit process gas inlet line, the evaporator coolant exhaust is connected to the deep-freeze unit coolant inlet line, the deep-freeze unit coolant exhaust is connected to the precooler coolant inlet line, and the deep-freeze unit coolant exhaust and the precooler coolant exhaust are connected to one of the at least one evaporator heating medium inlet lines.
 8. The device in accordance with claim 7, wherein the evaporator heating medium exhaust connected via the evaporator heating medium inlet line to the deep-freeze unit coolant exhaust is connected to the precooler coolant inlet line.
 9. The device in accordance with claim 7, wherein a storage tank for a condensed gas is connected to the evaporator coolant inlet line.
 10. The device in accordance with claim 7, wherein at least one of the precooler and the deep-freeze unit has a condensate outlet line. 